Tropical forests emit more carbon each year than all of the cars and trucks in the United States, scientists said on Thursday, calling for greater efforts to stem forest loss and damage.

Almost 70 per cent of tropical forest emissions are caused by degradation, a study in the journal Science said, measuring the less visible form of damage for the first time along with deforestation, which has long been recognized as problematic.

"These findings provide the world with a wake-up call on forests," said the study's lead author, Alessandro Baccini, a scientist with the U.S.-based Woods Hole Research Center, in a statement.

Tropical forests play a key role in combating global warming, as they absorb carbon during photosynthesis. But they emit carbon when they burn or decay after dying.

'If we're to keep global temperatures from rising to dangerous levels, we need to drastically reduce emissions and greatly increase forests' ability to absorb and store carbon,' said the study's lead author. (Ulises Rodriguez/Reuters)

Deforestation accounts for 10 to 15 per cent of carbon emissions worldwide. It is doubly damaging as decomposing trees produce carbon and are no longer able to store it, Baccini told the Thomson Reuters Foundation.

"Throughout the tropics you have selective logging, or smallholder farmers removing individual trees for fuel wood," Wayne Walker, another of the authors, said in a statement. "These losses can be relatively small in any one place, but added up across large areas they become considerable."

Carbon losses from deforestation and forest degradation exceed gains on every continent, said the study, produced using 12 years of satellite imagery and field measurements.

Latin America, home to the Amazon, the world's largest rainforest, was responsible for nearly 60 per cent of carbon losses, followed by Africa at 24 percent, it said.